Triglycerides or triacylglycerols are the major form of energy storage in eukaryotic organisms. In mammals, these compounds are primarily synthesized in three tissues: the small intestine, liver and adipocytes. Triglycerides or triacylglycerols support the major functions of dietary fat absorption, packaging of newly synthesized fatty acids, and storage in fat tissue (see Subauste and Burant, Current Drug Targets—Immune, Endocrine & Metabolic Disorders (2003) 3, 263-270).
Diacylglycerol O-acyltransferase, also known as diglyceride acyltransferase or DGAT, is a key enzyme in triglyceride synthesis. DGAT catalyzes the final and rate-limiting step in triacylglycerol synthesis from 1,2-diacylglycerol (DAG) and long chain fatty acyl CoA as substrates. Thus, DGAT plays an essential role in the metabolism of cellular diacylglycerol and is critically important for triglyceride production and energy storage homeostasis (see Mayorek et al, European Journal of Biochemistry (1989) 182, 395-400).
DGAT has a specificity for sn-1,2 diacylglycerols and will accept a wide variety of fatty acyl chain lengths (see Farese et al, Current Opinions in Lipidology (2000) 11, 229-234). DGAT activity levels increase in fat cells as they differentiate in vitro and recent evidence suggests that DGAT may be regulated in adipose tissue post-transcriptionally (see Coleman et al, Journal of Molecular Biology (1978) 253, 7256-7261 and Yu et al, Journal of Molecular Biology (2002) 277, 50876-50884). DGAT activity is primarily expressed in the endoplasmic reticulum (see Colman, Methods in Enzymology (1992) 209, 98-104). In hepatocytes, DGAT activity has been shown to be expressed on both the cytosolic and luminal surfaces of the endoplasmic reticular membrane (see Owen et al, Biochemical Journal (1997) 323 (pt 1), 17-21 and Waterman et al, Journal of Lipid Research (2002) 43, 1555-156). In the liver, the regulation of triglyceride synthesis and partitioning, between retention as cytosolic droplets and secretion, is of primary importance in determining the rate of VLDL production (see Shelness and Sellers, Current Opinions in Lipidology (2001) 12, 151-157 and Owen et al, Biochemical Journal (1997) 323 (pt 1), 17-21).
Two forms of DGAT were cloned and designated DGAT1 and DGAT2 (see Cases et al, Proceedings of the National Academy of Science, USA (1998) 95, 13018-13023, Lardizabal et al, Journal of Biological Chemistry (2001) 276, 38862-38869 and Cases et al, Journal of Biological Chemistry (2001) 276, 38870-38876). Although both enzymes utilize the same substrates, there is no homology between DGAT1 and DGAT2. Further, although both enzymes are widely expressed, differences exist in the relative abundance of DGAT1 and DGAT2 expression in various tissues.
The gene encoding mouse DGAT1 has been used to create DGAT knock-out mice. These mice, although unable to express a functional DGAT enzyme (Dgat−/− mice), are viable and continue to synthesize triglycerides (see Smith et al, Nature Genetics (2000) 25, 87-90). This would suggest that multiple catalytic mechanisms contribute to triglyceride synthesis, such as DGAT2. An alternative pathway has also been shown to form triglycerides from two diacylglycerols by the action of diacylglycerol transacylase (see Lehner and Kuksis, Progress in Lipid Research (1996) 35, 169-210).
Dgat−/− mice are resistant to diet-induced obesity and remain lean. When fed a high fat diet, Dgat−/− mice maintain weights comparable to mice fed a diet with regular fat content. Dgat−/− mice also have lower tissue triglyceride levels. The resistance to weight gain seen in the knockout mice, which have a slightly higher food intake, is due to an increased energy expenditure and increased sensitivity to insulin and leptin (see Smith et al, Nature Genetics (2000) 25, 87-90, Chen and Farese, Trends in Cardiovascular Medicine (2000) 10, 188-192, Chen and Farese, Current Opinions in Clinical Nutrition and Metabolic Care (2002) 5, 359-363 and Chen et al, Journal of Clinical Investigation (2002) 109, 1049-1055). Dgat−/− mice have reduced rates of triglyceride absorption, improved triglyceride metabolism, and improved glucose metabolism, with lower glucose and insulin levels following a glucose load, in comparison to wild-type mice (see Buhman et al, Journal of Biological Chemistry (2002) 277, 25474-25479 and Chen and Farese, Trends in Cardiovascular Medicine (2000) 10, 188-192).
Disorders or imbalances in triglyceride metabolism, both absorption as well as de novo synthesis, have been implicated in the pathogenesis of a variety of diseases risk. These include obesity, insulin resistance syndrome, type II diabetes, metabolic syndrome (syndrome X) and coronary heart disease (see Kahn, Nature Genetics (2000) 25, 6-7, Yanovski and Yanovski, New England Journal of Medicine (2002) 346, 591-602, Lewis et al, Endocrine Reviews (2002) 23, 201, Brazil, Nature Reviews Drug Discovery (2002) 1, 408, Malloy and Kane, Advances in Internal Medicine (2001) 47, 111, Subauste and Burant, Current Drug Targets—Immune, Endocrine & Metabolic Disorders (2003) 3, 263-270 and Yu and Ginsberg, Annals of Medicine (2004) 36, 252-261). Compounds that can decrease the synthesis of triglycerides from diacylglycerol by inhibiting or lowering the activity of the DGAT enzyme would be of value as therapeutic agents for the treatment diseases associated with abnormal metabolism of triglycerides.
Known inhibitors of DGAT include: dibenzoxazepinones (see Ramharack, et al, EP1219716 and Burrows et al, 26th National Medicinal Chemistry Symposium (1998) poster C-22), substituted amino-pyrimidino-oxazines (see Fox et al, WO2004047755), chalcones such as xanthohumol (see Tabata et al, Phytochemistry (1997) 46, 683-687 and Casaschi et al, Journal of Nutrition (2004) 134, 1340-1346), and substituted benzyl-phosphonates (see Kurogi et al, Journal of Medicinal Chemistry (1996) 39, 1433-1437, Goto, et al, Chemistry and Pharmaceutical Bulletin (1996) 44, 547-551, Ikeda, et al, Thirteenth International Symposium on Athersclerosis (2003), abstract 2P-0401, and Miyata, et al, JP 2004067635).
Also known to be inhibitors of DGAT are: 2-bromo-palmitic acid (see Colman et al, Biochimica et Biophysica Acta (1992) 1125, 203-9), 2-bromo-octanoic acid (see Mayorek and Bar-Tana, Journal of Biological Chemistry (1985) 260, 6528-6532), roselipins (see Noriko et al, (Journal of Antibiotics (1999) 52, 815-826), amidepsin (see Tomoda et al, Journal of Antibiotics (1995) 48, 942-7), isochromophilone, prenylflavonoids (see Chung et al, Planta Medica (2004) 70, 258-260), polyacetylenes (see Lee et al, Planta Medica (2004) 70, 197-200), cochlioquinones (see Lee et al, Journal of Antibiotics (2003) 56, 967-969), tanshinones (see Ko et al, Archives of Pharmaceutical Research (2002) 25, 446-448), gemfibrozil (see Zhu et al, Atherosclerosis (2002) 164, 221-228), and substituted quinolones (see Ko, et al, Planta Medica (2002) 68, 1131-1133).
A need exits in the art, however, for additional DGAT inhibitors that have efficacy for the treatment of metabolic disorders such as, for example, obesity, type II diabetes mellitus and metabolic syndrome. Further, a need exists in the art for DGAT inhibitors having IC50 values less than about 1 μM.